Try to fix browser tests, close#612

[oyyd]

Due to attempt to modify read-only property length -- see http://mozilla.github.io/nunjucks/files/tests/browser/

It's the problem of expect.js so that I have updated the newest version in /tests/browser.

I also updated the newset version of mocha in /tests/browser as it will cause a timeout failure in browsers.

But there are still two failing tests related to ignore missing which was not tested in browsers before and I don know how to do with them.

[carljm]

Thank you! This is a great pull request, much appreciated.

The ignore missing tests are failing because the WebLoader throws an error instead of returning null on a missing template. I'll merge this and then fix that separately.

[carljm]

FYI I fixed the WebLoader issue and pushed v2.3.0, and the online docs now have passing tests: http://mozilla.github.io/nunjucks/files/tests/browser/

Thanks again!

[oyyd]

@carljm 👍 Good job